1. Field of the Invention
The present invention relates to a method for forming a semiconductor device, and more particularly to a method for forming a contact window.
2. Description of Related Art
Non-volatile memory device allows multiple and repetitive writing, reading and erasure operations, and the stored data is retained even when power is cut off. Hence the non-volatile memory device has become widely used in personal computers and electronic equipments.
A typical non-volatile memory device includes a plurality of select transistors and a plurality of memory cells. In general, the memory cell is designed to have a stacked-gate structure, and the stacked-gate structure includes a tunneling dielectric layer, a floating gate, an inter-gate dielectric layer and a control gate. The select transistor generally has a select gate and a gate dielectric layer. In order to electrically connect the select transistors to bit lines, an opening is usually formed between the select transistors and the opening is filled with a conductive material to form a contact window, thereby electrically connecting the select gates of the select transistors to the bit lines.
Nonetheless, a common way to increase storage capacity of a memory device is to increase the number of memory cells per unit of volume in the memory. However, the increased number of memory cells causes increased component density in the memory. With respect to an NAND gate memory, distance between the select gates becomes smaller and smaller. When the contact window is formed between the select gates, there is need to form the contact window opening with a large height-width ratio; in cases where conventional methods for forming a contact window are used, a contact window opening having an under-cut or side-etch/bowing profile may be obtained, and consequently, a major axis direction of the contact window is not perpendicular to a surface of a substrate to be electrically connected. Such under-cut or side-etch/bowing profile may result in formation of seams in some portions in the contact window opening which are not filled with tungsten after a step of filling the tungsten in the contact window opening during the subsequent fabrication processes. As a result, a resistance of the contact window is enhanced, and adverse effects arise. In addition, with reduced device density, the contact window having a side-etch/bowling profile may cause short circuits between adjacent select gates or bit-line contact windows.